In the other room, the boy loosens up with some old Peter, Paul, and Mary tunes. ``Five Hundred Miles,'' ``A 'Soalin.''' In a leap of generations, he moves on to Led Zeppelin and ``Stairway to Heaven.'' Then it's back to folk picking, ``House of the Rising Sun.'' Finally, he settles down to the serious scales and chord progressions his guitar teacher will want to hear tomorrow - with a break for Neil Young's ``Needle and the Damage Done.'' The listener wonders why this 13-year-old picks out the dark tunes, but decides not to worry about it. The boy had his new braces installed today, the heavy metal bands of adolescent life, and the music helps soothe the discomfort. Who says a kid can't be a blues man? Thirty years ago, his father sat in a room like that. Sometimes he still does.